1. Field of the Invention
The invention relates to a method for producing image sensors on the basis of TFA technology, comprising an amorphous thin-layer system that has been applied on a crystalline ASIC.
2. Discussion of the Related Art
This TFA technology (thin film on ASIC) is known from B. Schneider, P. Rieve, M. Böhm, Image Sensors in TFA (Thin Film on ASIC) Technology, ed. B. Jähne, H. Hausecker, F. Geiβler, Handbook of Computer Vision and Applications, pp. 237-270, Academic Press, San Diego, 1999. Thin-layer systems, which can be based on amorphous silicon, for example, and which have been applied on a crystalline ASIC (application specific integrated circuit), convert incident light into a photocurrent, which is further processed by the underlying ASIC. The optoelectronic advantages of the amorphous material are combined with the electrical advantages of the crystalline material and utilized. The thin-layer system can generally be structured on any crystalline ASIC without any problem, with ASICs based on known CMOS technology (complementary metal oxide semiconductor) being especially suited. The circuits necessary for image sensor operation are then realized in the ASIC.
The problem encountered in conventional ASICs is that high leakage currents arise in the crystalline components. These leakage currents become superposed on the photocurrents, and vary from pixel to pixel as well as with temperature.
Since these leakage currents are also converted into signals by the other evaluation circuits of the ASICs, just as the photocurrents, the leakage currents cause spurious signals to be superposed on the useful signals, which become visible in the image as transient noise and fixed noise (also called FPN, fixed pattern noise) in particular given longer integration times and lower luminous intensities, thereby detracting from image quality.
By contrast, the level of leakage current contributed by the thin-layer system is often far lower, and would in and of itself offer a better image quality at low luminous intensities.